1. Field of the Invention
The subject invention generally pertains to centrifugal compressors, screw compressors and other types of compressors of hermetically sealed refrigerant systems and more specifically to a means for protecting the motor bearings and seals of such compressors.
2. Description of Related Art
For years, it has been known that adverse shaft voltage can develop between the shaft and housing of electric motors. When the voltage discharges with sufficient energy across a bearing that supports the shaft, the resulting current can damage the bearing through electrical arcing and/or by degrading the bearing's lubricant.
The shaft voltage can actually be a combination of voltages originating from different sources. It can be electrostatically generated within the motor, or the voltage can arise from imbalanced ampere-turns in the stator or from stator or rotor asymmetries. In cases where the motor is driven by an AFD (adjustable frequency drive) with high speed power switching devices (e.g., insulated gate bipolar transistors), the rapid switching rate of the power switching devices can generate a high frequency common mode voltage that can ultimately lead to a damaging component of the shaft voltage.
Attempts at solving the shaft voltage problem have involved various means such as shaft-grounding brushes, electrically insulated bearings, ceramic bearings, low impedance lubricant, common mode inductors or line filters applied to the motor's AFD.
Although such solutions may be effective for a typical motor driven by an AFD, centrifugal refrigerant compressors do not fit this simple model. Centrifugal refrigerant compressors are unique in that the motor and the compressor share a common housing. With a shared common housing, some mixing of the lubricant and the refrigerant can occur, which can alter the dielectric and/or other properties of the lubricant. Also, various properties of the housing and even the quantity and location of the impellers within the housing seem to have an effect on the induced shaft voltage.
Since centrifugal refrigerant compressors do not fit the typical inverter/motor model, it has been painfully discovered that previous shaft voltage mitigating solutions are not always successful when applied to refrigerant-based systems. For centrifugal refrigerant compressors, solutions that have worked in the past are often found to be unsuccessful today, which suggests that something has changed.
Identifying which particular change or changes are making present shaft voltage problems more difficult to correct is very challenging. Some of the variables may include new refrigerants, new lubricants, new bearings, different refrigerant seals, and the style of the compressor housing as it relates to the quantity and layout of the compressor's impellers.
To fully correct the shaft voltage problem, a need exists for a total solution or a set of solutions that can be successfully applied to centrifugal refrigerant compressors of various designs and configurations.